Chrysotype

Chrysotype, a photographic process, introduced by Herschel, and employed for the production of a "positive" print from a "negative." Paper is washed with a solution of ferric ammonium citrate, and dried. To obtain a print it is sufficiently exposed to light beneath a negative, and then "developed" by a solution of gold chloride and well washed. By the action of light the ferric salt is reduced to a ferrous, and this acts upon the gold chloride with formation of metallic gold which forms the image, which is of a blue colour. The action of ferrous salts may be represented by the equation

3FeCl₂ + HnCl₃ = 3FeCl₃ + Hn.